1. Field of the Invention
The field of the invention is that of gas turboshaft engines for aircrafts. The invention relates more particularly to a system for retaining a fan of a turboshaft engine in the event of breakage of a driving shaft of the fan.
2. Description of the Related Art
In an emergency situation, it is necessary to protect the engine so that the aircraft can be diverted towards the closest airport. In the event of partial or total loss of a fan blade, the fan goes unbalanced and oscillates. This phenomenon is known to the person skilled in the art under the designation “fan imbalance” (orbitage de la soufflante in French). These oscillations generate important stresses which can cause collateral damage. To ensure safety and avoid such damage, it is necessary to take these stresses into account when sizing the structures of the engine and airplane, which makes the engine heavier than a standard engine which would be designed to bear stresses during normal flights only. To limit the dynamic loads in the fan and relieve the engine, a decoupling device which makes it possible to limit the stresses at high RPM is known from patent application FR 2 877 046 by SNECMA company. In practice, in a twin-shaft turboshaft engine comprising a low-pressure shaft and a high-pressure shaft, the decoupling system is arranged so as to decouple the bearings that bear the low-pressure shaft for driving the fan with respect to the fan frame, these bearings being known to the person skilled in the art under the designation “bearing 1” and “bearing 2”.
After the fan is decoupled, the modal situation of the engine is described as “hypercritical”, the only mode of operation of the engine is then low RPM. In an advantageous way, the dynamic loads in the fixed structures are then very much reduced at high RPM. At high RPM, the dynamic loads in the shaft remain important and the risk of breakage of the low-pressure driving shaft increases.
In reference to FIG. 1, a twin-shaft engine 1 has a low-pressure shaft 2 solidly connected to a fan 4 and a high-pressure shaft 3 which rotate around an axis X of the engine. Thereafter, the “upstream” and “downstream” terms are defined with reference to the movement of the gases in the engine, the gases moving from upstream to downstream. Similarly, in the present application, the terms “inner” and “outer” are conventionally defined radially with reference to the axis X of the engine shown in FIG. 1. So, a cylinder extending according to the axis of the engine has an inner surface facing the axis of the engine and an outer surface opposite to its inner surface.
Still in reference to FIG. 1, the low-pressure shaft 2 is rotatively guided in a fan frame 5 of the engine 1 by means of bearings P1, P2 known to the person skilled in the art under the designation “bearing 1” and “bearing 2” respectively. In reference to FIG. 1, the fan frame 5 has an outer casing 57 and an inner hub 58 connected by means of structural struts 59. The high-pressure shaft 3 is rotatively guided in the fan frame 5 of the engine 1 by means of a bearing P3 known to the person skilled in the art under the designation “bearing 3”. As shown in FIG. 1, the engine 1 includes a forward fairing 15, also called “splitter”, solidly connected to the hub 58 of the fan frame 5 and arranged to guide, internally, a core-engine flow and, externally, a fan flow. Besides, the engine 1 has a low-pressure compressor, known to the person skilled in the art under its English designation “booster”, which has stator blades 62 solidly connected to the forward fairing 15 and mobile blades 61 solidly connected to a driving drum 6 in order to compress the core-engine flow moving from upstream to downstream in the engine 1. A decoupling device 10 is arranged between the hub 58 of the fan frame 5 and the bearings P1, P2 so as to make the structure of the engine more flexible in the hypercritical mode as explained previously.
In the event of breakage of the low-pressure shaft 2 downstream from the bearing P2, there is a catching device 20, known under its English designation “catcher”, which makes it possible to axially retain the upstream part of the low-pressure shaft 2 solidly connected to the fan 4. This catcher 20 includes on the one hand a ring 21 extending radially inwards from the hub 58 of the fan frame 5 and on the other hand an annular rim 22 extending radially outwards from the low-pressure shaft 2. So, in the event of breakage of the low-pressure shaft 2 downstream from the bearing P2, the part of the low-pressure shaft 2 which is situated upstream from the zone of breakage moves upstreamwards so that the annular rim 22 comes into contact with the ring 21 and retains it axially. In other words, the upstream part of the low-pressure shaft 2 is “caught” by the catcher 20 in the event of breakage of the low-pressure shaft 2.
Such a catcher 20 is satisfactory in the event of breakage of the low-pressure shaft 2 downstream from the bearing P2 but is ineffective in the event of breakage upstream from the said bearing P2. So, the stator blades 62 are not capable of retaining the mobile blades 61 of the drum 6 in the event of breakage of the low-pressure shaft 2.